1. Field of the Invention
The present invention relates to bipolar high output current buffers, and more particularly to buffers with stacked input and output transistors.
2. Background Information
Prior art bipolar high current buffers typically comprise four transistors, a PNP and NPN input and a corresponding NPN and PNP output arranged in a push pull or the other stacked configuration. In this configuration, the output is limited by the output transistor beta (current gain) and the base drive available from the input stage. This leads to higher than desired quiescent power dissipation, due to high current in the input stage. FIG. 1 shows such a buffer, where an input signal (IN) is applied to the bases of Q2 and Q3, and the output(OUT) is taken from the emitters of Q1 and Q4. Notice that the current available to the base of Q1 is at most I2, and the current available to the base of Q4 is at most I1. Notice also, that the output voltage swing can come within one base emitter drop of Vcc, when high, and one base emitter drop of Vee, when low.
One prior art approach to reduce the quiescent power dissipation is to use another set of transistors to in effect increase the current gain by ananging an emitter of another transistor to drive the base of the output transistor. However, such a circuit will reduce the dynamic swing of the output by at least one additional base emitter drop near the Vee rail and another near the Vcc rail. FIG. 2 illustrates this approach. Notice that Q5's base drive is sourced from the emitter of Q7. The range, in this circuit, of available output current via Q5 is the current source I3 multiplied by the beta of Q7 times the beta of Q5. So, large output currents are available even with a small I3 current. Since I3 is small, the quiescent power dissipation in the circuit is much smaller than that of FIG. 1. However, notice that if Q7 is fully on, Vee plus a Vbe (base emitter drop) appears at Q7's emitter and so at the base of Q5, and that the OUT is necessarily higher than Vee by the two Vbe base emitter drops (Q5 and Q7). The same is true at the Vcc end of the dynamic range where I4 drives the base of Q8, and, where the OUT is at least the base emitter drops of Q6 and Q8 below Vcc. This limits the dynamic swing of the circuit in FIG. 2 to at least four Vbe's less than Vcc to Vee.
U.S. Pat. No. 4,574,233 to Taylor (Taylor) describes a high impedance current source using negative feed back to produce a high impedance current source that operates with small voltages (tens/hundreds of millivolts) across the current source. The circuit is shown in FIG. 3. The output current is the collector current of Q11 and any change is sensed across the resistor R2. The sensed voltage across R2 appears across R1 via the common base connections of Q15 and Q17. The current through the collector of Q15 changes and that in turn changes the base current to Q11 in such way to reduce the original change (that is negative feedback). The circuit of FIG. 3 provides a high output impedance current source output that operates close to the Vee rail. Taylor also teaches that the ratio of R2 to R1 can fix the loop gain of the feed back loop and is selected to produce a relatively high incremental output impedance, see column 2, line 50.
Taylor in column 2, lines 31-36, describes how his high output impedance current source circuit of FIG. 3 can be operated as an amplifier. The input is a voltage at the collector of Q15 and the output is taken from the collector of Q11. The present invention makes use of a circuit similar to that shown in FIG. 3, but with some differences. Those differences are not shown, anticipated, taught or suggested by Taylor.